Extraction of cyanidable precious metals from carbonaceous ores



alkaline earth metal mercaptides.

S ates EXTRACTION F .CYANIDABLE PRECIOUS METALS FROM CARBONACEOUS ORESNo Drawing. Application January 22', 1951 Serial No. 635,136,

Claims. (Cl. 75 -105) This invention relates to an improved method ofrecovering precious metals from carbon-containingqores. The cyanidationofprecious metal ores, suchas gold and silver ores, is the standardmethod for recovering the precious metal values and under suitableconditions is highly efiicient. Howeventhere arezores which pre? sentserious problems. One type of ore is that containing carbon orcarbonaceous impurities. Apparently, the carbon causes a reprecipitationof the dissolved gold and/ or silver from their complex cyanidesolutions. As a result when such ores are cyanided, recoveriessuffer andin some cases cyanide consumption increases. As a result thesecarbonaceous ores were for many years considered as unsuitableforcyanidation. H Attempts have been made to overcome the difficulties bythe addition of certain materials. For example, min: eral oils, such asfuel oil, were described intheDorfman Patent 1,411,326. The patentee'wasof the opinion that these mineral oils were adsorbed on the carbonaceousmaterial and so presumably insulated it from contact with the dissolvedprecious metal complex cyanide. The hydrocarbon oil does, in fact,improve extraction, whether by reason of the suggested mechanism or forsome other reasons, butthere is a limit to the improvement which may beobtained and extractions of previous metal still leave much to bedesired, although'thefueloil' does help and as will be pointed out belowmaybe used, in

phoric'acids which also exist as their salts in the strongly alkalinecyanidation pulps. The aryl radicals in the diaryldithiophosphoric acidmay contain alkyl substituenta In,the.case of'this type of compound,however, there is a much greater tolerance for aliphatic carbon atomsbuthere again there is a very definite limit. Thealiphatic.

carbon atoms must be not more than twice the number of'aromatic carbonatoms. 1 @The third classare diarylthioureas. Here again there is alimited tolerance for aliphatic carbon atoms but'there should bemorefaromatic. carbon atoms than aliphatic carbon atoms, the carbon atomof the'thiourea itself being consideredas neutral. 1 1

1 The, fourth type of compound is a mercaptothiazole.- '1-The fifth typeof compound is an alkyl phenol, but herethe presence of alkyl carbonatoms is essential, for phenol itself is not elfective, and there doesnot appear to be any limitation on the number of aliphatic carbonatoms,monoand polyalkyl phenols being very eifective. Z The amount ofreagent used is not critical and ranges from a small fraction of a poundper ton of ore up to several pounds per ton. In the case of many of thecompounds and with certain ores, amounts from 1 to 2 pounds per ton arevery efiective. Larger am'ounts may be used but nobenefit is obtainedwhen the amount greatly exceeds 2 or 3 pounds per ton. Consequently, itis desirable to use as littlereagent as will givegood'em. tractions.Normally, however, the reagents are sufiiciently cheap so that a smallexcess over the absolute minimum for any given ore is desirable to takecare of th operating variations. I

It is in no sense necessary that compounds of only one of the fiveclassesbe used. On the contrary, mixtures of more than one compound inthe class, or of compounds belonging to more than one class, areequally.

conjunction with the other reagents to which the present inventionrelates. I 1

Another solution to the problem is described in. the Booth Patent2,220,034 in which certain dyes are used. This process is a markedimprovement and has been successfully used onu some carbonaceous ores...The price of the dyes, however, is quite high, and there was there forestill left a demand for an equally efiicient' or more an exit processwhich use sfcheapermaterial. e present invention'utilizesreagentsbelongingto five, diifer'ent groups of organic chemicals. The.rnechais not entirely understood, audit is not known w hy the differentchemical classes operate. It is possible that there is somecommon'characteristic or mechanism of reaction. Itis also possible thateach group behaves in a difierent manner. Except for the fact that thedifferent groups are affective in producingbetter precious metalextraction, it is not known that they '"are in any sense equivalents oroperate by the same reaction mechanism. The first group ofchemicalcompounds used in the present invention are aralkyl mercaptanswhich, of course, may exist in the normally strongly alkalinecyanidation pulps partly or wholly as the corresponding alkali or Thisgroup of compounds must have a larger number of aromatic carbon atomsthan aliphatic carbon atoms. This is quite critical but the reasonfol-this behavior is not known and, ac cordingly, this invention is notlimited to. the theory of why the reagents are sensitive to an excessiveamount of aliphatic carbon atoms.

effective and with certain refractory ores occasionally show someadvantage. It is also possible to use the hydrocarbon oils, such aslight fuel oils, with the reagents. of the present invention. In somecases this permits the. use of a somewhat smaller amount of the moreexpensive. chemical. In other'cases, for example with some veryrefractory ores, the result of the mixture is actually better. Thepresent invention therefore contemplates the use of compounds belongingto one or more of the five classes .enumerated above with or withoutfuel oil,-or-

other-hydrocarbon oil.

It is a very real advantage of the present invention that the reagentsmaybe very crude. Within wide limits purity of the compound does notappear to be of any importance. It is thus possible to use crudecompounds such asfiotation-grade dithiophosphates, diarylthioureas, etc.These crude compounds are much cheaper than pure chemicals and, becausethey are just as effective, they are preferred for economical reasons.

The present invention is not limited to the use of inorganic cyanides inconjunction with the reagents of the five classes. On the contrary,excellent results are obtained when the cyanidation reagent is analpha-hydroxynitrile, such as lactonitrile, mandelonitrile and the like.

Very crude alpha-hydroxynitriles may be used such as the crudelactonitrile obtained as a dirty brown byproduct from the production ofacrylonitrile.

It should be noted that the aromatic groups in the M substitutedthioureas need not in all cases be linked to the nitrogen of'thethiourea from a nuclear carbon atom.

Aralkyl groups such as benzyl are equally effective. It; should be notedalso that particularly in the case. of the modified mercaptans, thepresence of inorganic u st t at l in e' v groups, su h a a qsenssi n t.

The second class of compounds are diaryldithiophov interfere with theefiectiveness of the reagent. How- 3 ever, in general these inorganicsubstituents should be covalently linked, rather than electrovalentlylinked.

The invention will be described in conjunction with the specificexamples which are typical in carbonaceous ores which are not amenableto ordinary cyanidation. In each case the ore was ground to the finenessspecified in the example, diluted with water to a pulp of approximately25% solids, and subjected to cyanidation under otherwise standardconditions with 4.0 pounds per ton NaCN. In each case lime was added toproduce an alkaline pH for optimum extraction. The pHs were of the orderof 11. The other operations also follow standard cyanidation practice.In each case the pulp was filtered, washed and the solution then treatedto extract the precious metals and assayed. Unless otherwise specifiedcyanidation time was 24 hours.

Because of the strong alkalinity of the conventional cyanidation pHused, the acidic reagents, such as the dithiophosphoric acids and alsoperhaps in the case of the mercaptans and the mercaptothiazoles, theactual compound exists in the fonm of its alkali salt. In the claims,therefore, the diaryldithiophosphoric acid will be expressed as theanion as it exists in combination with whatever cation is used in thealkali to produce the pH in question. The term mercaptan is usedgenerically to include its salts.

Example 1 A Canadian ore from Canada assaying 0.311 oz. gold per ton and1.82% carbon was ground to 91% 200 mesh and portions were cyanided for48 hours. The results with the difiFerent added reagents and the blankappear in the following table.

A Canadian ore analyzing 0.313 02. gold per ton and 1.12% carbon wasground to 93% 200 mesh and portions cyanided for 48 hours. The resultswith different reagents are shown in the following table.

Reagent Used Residue Au, Extr. Au, Amt. ozJton Percent Nome Used,

lbJton Blank 0.288 4. Mixture of Dicrcsyldithiophosphoric acid andthiocarbanilide 2.0 0.060 81.1 p-Isopropylbenzylmercaptan 2. 0 0. 08573. 4 Dimethylbonzylmercaptan 2. 0 0. 066 79. 0Morcaptomcthyldimethylnaphthalena 2. 0 0. 099 70. 2 3,4-Dichlorhenzylmcrcaptan 2.0 0.078 76.1 Zincdioct.ylphenyldithiophosphatc. 2.0 0. 103 66. 9Triisopropylbcnzylmercaptan 2. 0 0. 217 31. 8 Dodccylbenzylmercaptan 2.0 0. 210 29. 3

It will be noted that alkylbenzylmercaptans having more alkyl carbonatoms than aromatic carbon atoms give very inferior results, althoughbetter than the blank alone.

The procedure of the above example was repeated using 4 lb./ton NaCNequivalent of crude lactonitrile with 2 1b./ ton of a mixture of onepart of a 6% solution ofthiocarbanilide in technical gradediscresyldithiophosphoric acid and one part of a crude aromatichydrocarbon oil. The residue contains 0.058 oz./ton Au and theextraction was 82.2%.

Example 3 A Canadian ore analyzing 0.437 oz. gold per ton and 1.33%carbon was ground to 98% 200 mesh and portions were cyanided for 48hours. The results appear in the following table.

Reagent Used Residue u, Extr. Au, Amt. oz./ton Percent Name Used,lb./ton

Blank 0. 270 36. 9 p-Isopropylbenzylmercaptau 2. 0 0. 055 88. 2 Mixtureof Dicresyldithiophosphoric acid and thiocarbanilide 2.0 0.050 89. 1Mixture of Dicresyldithiophosphoric acid and an aliphatic hydrocarbonsolvent 2.0 0.039 91. 2

Example 4 An ore from the Gold Coast analyzing 0.71 02. gold per ton and1.37% carbon was ground to 82% '200 mesh and portions cyanided. Theresults appear in the following table.

Reagent Used Residue Extr.

Au, Au, Amt. oz./ton Percent Name Used,

lb./ton

Blank 0. 185 73. 8 Dicresyldithio-phosphoric l. 0 0. 080 88. 4 Mixtureof Dicresyldithio-phosp acid and thio-carbanilide 1. 0 0.076 88. 9Mixture of Dicresyldithioacid and di-o-tolylthiourea 1. 0 0.078 88.7Ammonium dicresyldithio-phosphate. 1. 0 0.087 87.3 Mixture of Ammoniumdicresyldithiophosphate and thiocarbanilide 1. 0 0. 84. 7 Sodiummercaptobenzothiazole 1. 0 0.128 81. 4 Thiocarbanilide 1. 0 0. 085 87. 6Di-o-tolylthiourea 1. 0 0. 086 87. 3 Mixture of Thiocar de and Fuel 011N0. 2 2.0 0.071 89. G Mixture of Dicresyldithiophosphoric acid and FuelOil N0. 2. 2.0 O. 058 91. 6 Diethyldithiophosphoric aci 1. 0 0. 210 69.4 2-Benzothiazolyldisulfide 1. 0 0. 102 85. 2 Morpholine Salt ofmercaptobenzothiazole 1. 0 0. 074 89. 2 Xylenol 1. 0 0. 093 8G. 46o-Amyl phenol- 1. 0 0.08 88. 36 Di(tertiarybutyl)ereso 1. 0 0. 075 89.20 Diamyl phenol. 1. 0 0. 069 89. 96 N onyl phenol- 1. 0 0. 068 90. 39Oresylic acid 1. 0 0. 082 88. 06 Mixture 46% No. 2 Fuel Oil, 46 0 acid,8% thiocarbanilide. 3.0 0.070 89. 81 Phenol 1. 0 0. 143 79. 18

It will be noted that in the case of the phenols, the presence of alkylcarbon atoms is essential as phenol itself gave no useful'result. Thesame is true of diethyldithiophosphoric acid which was actually worsethan the blank.

Example 5 An ore from the Gold Coast analyzing 1.01 oz. gold per ton and0.86% carbon was ground to 74% -200 mesh and portions were cyanided. Theresults appear in the following table.

Reagent Used Residue Au, Extr. Au, Amt. ozJton Percent Name Used,

lb./ton

Blank 0. 105 89. 6 Mixture of Dicresyldithiophosphoric acid andthiocarbanilide 1.0 0.093 90.8 Sodium mercaptobenzothiazole. 0. 5 0. 06093.1 Di(2-naphthyl)dithiophosphorie acid and thiocarbanilide 2.0 0. 03896. 0 Mixture of Dicresyldithiophosphoric acid and Fuel 011 N0. 2 2. 0o. 045 95. 4

asamae Example 6 An ore from the Gold Coast analyzing 2.43 oz. gold perton and 0.72% carbon was ground to 74% 200 mesh and portions cyanided.The results appear in Example 8 Another ore from the Gold Coastanalyzing 0.285 oz. gold per ton and 0.38% carbon was ground to 84% thefollowing table -200 mesh and portlons cyanided for 48 hours.

Reagent Used Reagent Used Residue Residue Au, Extr. Au, Au, Extr. Au,Amt. ozJton Percent 10 Amt. ozJton Percent Name Used, Name Used, lb./tonlb./ton

Blank 0. 163 93. 3 Blank o. 025 89. 9 Mixture ofDicresyldithiophosphoric DlQ'naphthyI)dithiophosphoric acid.-. 2.0 0701892.7

acid and thioearbanllide 1.0 0.103 5 8 Mlxture ofDicresyldithiophosphorie acid and thioearbanillde 2.0 0. 019 93. 0

This example shows that even in the case of an ore which contains carbonand yet is readily cyanided an improvement is made by the use of atypical reagent of the present invention; although, of course, here thedifference is much smaller than with the more refractory ores. In thisrespect the results are not greatly different from Example 5, which alsodealt with an ore which is fairly amenable to cyanidation.

Example 7 A Canadian ore analyzing 0.098 02. gold per ton and 0.97%carbon was ground to 87% 200 mesh and portions cyanided for 48 hours.The results appear in the following table.

This ore illustrates the fact that with an extremely refractory ore inwhich ordinary cyanidation gives practically no extraction at all,improvement is obtained and this improvement is greater when thereagents of the present invention are combined with hydrocarbon oils.

This ore which is extremely low in carbon and is amenable to ordinarycyanidation shows that the reagents of the present invention are capableof producing improvement even where so high an extraction can beobtained in ordinary cyanidation procedure.

We claim:

1. A process of extracting cyanidable precious metals from carbonaceousores containing the same which comprises eifecting cyanidation in thepresence of an amount of a diaryldithiophosphoric anion having less thantwice as many aliphatic carbon atoms as aromatic carbon atoms sufficientto minimize the reprecipitating effect of the carbon and recovering theextracted precious metals.

2. A process according to claim 1 in which the diaryldithiophosphoricanion is dicresyldithiophosphoric anion.

3. A process according to claim 1 in which the diaryldithiophosphoricanion is di(2-napl1thyl)dithiophosphoric anion.

4. A process according to claim 1 in which the cyanidation agent is analpha-hydroxynitrile.

5. A process according to claim 4 in which the alphahydroxynitrile iscrude lactonitrile.

References Cited in the file of this patent FOREIGN PATENTS 554,290Great Britain June 28, 1943

1. A PROCESS OF EXTRACTING CYANIDABLE PRECIOUS METALS FROM CARBONACEOUSORES CONTAINING THE SAME WHICH COMPRISES EFFECTING CYANIDATION IN THEPRESENCE OF AN AMOUNT OF A DIARYLDITHIOPHOAPHORIC ANION HAVING LESS THANTWICE AS MANY ALIPHATIC CARBON ATOMS AS AROMATIC CARBON ATOMS SUFFICENTTO MINIMIZE THE REPRECIPATING EFFECT OF THE CARBON AND REMOVING THEEXTRACTED PRECIOUS METALS.